The rabbet-edge on ceiling tile is in the form of a recess which overlaps the companion recess of an abutting tile so as to cover the jointure more effectively. In addition, it provides support for each overlapping segment so as to hold the tiles more firmly in place.
Although ceiling tile is manufactured with a rabbet-edge it is often necessary during installation to conform same to the area which is to be covered as a result of which it is necessary to introduce a new edge.
Heretofore, several fresh-bladed knives were needed for this operation. Moreover, precise measurements were required so that the resulting tiles could be properly mated and thus provide a unitary appearance which avoided the old-fashioned block-look.
The present invention overcomes these difficulties by providing a tool with an adjustable blade for forming a perfect rabbet-edge in all instances.